Optical fibers have been used in telecommunication and medical applications for many years. The materials commonly used for making these fibers are glass or quartz; that is silica (SiO2). The silica core optical fiber has been used in telecommunication industry as transmission medium and medical field as light guide. However because of the limitation of the material property, silica core fiber can only transmit light up to 1.6 μm which may limit many of its applications in the biomedical and sensor fields.
There are generally two methods to make optical fibers. The rod-in-tube method is implemented by sleeving a glass rod inside glass tube, and then drawn them into fiber. The other method is a two step process of first making a preform and then drawing the preform into fiber. The second method is widely used in the industry because it allows the designer to custom make the desired properties of the produced product while also making a better quality fiber.
There are four process techniques generally being used for making silica core preforms: Outside Vapor Deposition (OVD) developed at Coring, taught in U.S. Pat. No. 3,737,292 and others; Vapor Axial Deposition (VAD) developed at NTT Laboratories, Japan and used by the major Japanese producers taught in U.S. Pat. No. 4,062,665 and others; Modified Chemical Vapor Deposition (MCVD) developed at AT&T Bell Laboratories and taught in U.S. Pat. No. 4,217,027 and others; as well as the Plasma Chemical Vapor Deposition (PCVD) developed at Philips Research Laboratories, Germany and taught in U.S. Pat. Re 30,635 and U.S. Pat. No. 4,741,747.
The OVD and VAD methods are very similar to each other. The difference is that OVD has the deposition target placed horizontally and VAD has the target in vertical position and moving upward during the process. The deposition target of both processes do not become part of the finished product and there may not be a limitation on the size of finished product. This allows these two processes to be very efficient manufacturing processes. However, these processes are very complicated and capital investments requirements are very high. In addition, both processes use hydrogen and oxygen torch as the energy source, which requires additional drying steps in making conventional optical fibers.
The MCVD and PCVD methods use tubes as a substrate to deposit new layers onto the inner wall of the substrate tube, where that tube becomes part of the finished product. The tube provides a clean and isolated environment for the new deposited materials, but it also limits the size of the preform can be made. The capital investments are much lower than OVD and VAD processes, and the processes are less complicated. MCVD method also uses hydrogen and oxygen torch as the energy source; the PCVD method uses microwave as its energy source, it allows wilder deposition temperature range and has higher material utilization rate.